The present invention relates to a process for converting trimethyl-hydroquinone diacetate (TMHQ-DA) into trimethylhydroquinone-1-monoacetate (TMHQ-1-MA) by contacting TMHQ-DA with a lipase to effect an enzymatic monosaponification of the TMHQ-DA. Methods of making (all-rac)-xcex1-tocopherol and (all-rac)-xcex1-tocopherol acetate are also provided.
The major commercial form of vitamin E is its acetate derivative, synthesized by acetylation of (all-rac)-xcex1-tocopherol, e.g. with acetic anhydride.
Industrial syntheses of (all-rac)-xcex1-tocopherol are based on the condensation of trimethylhydroquinone (TMHQ) with isophytol, phytol or a derivative thereof, such as a phytyl halide. TMHQ is normally obtained from 2,3,6-trimethylphenol which is expensive, however, and acidic catalysts have to be used for the condensation of the TMHQ with isophytol, phytol or a derivative thereof, such as a phytyl halide.
Alternatively, (all-rac)-xcex1-tocopherol acetate can be synthesized by condensing trimethylhydroquinone-1-monoacetate (TMHQ-1-MA) with isophytol or an equivalent thereof, i.e. e.g. phytol or a derivative thereof, such as a phytyl halide. The TMHQ-1-MA used in this alternative synthesis can be obtained from the much less expensive xcex1-isophorone via ketoisophorone and trimethylhydroquinone diacetate (TMHQ-DA), the latter having to undergo an absolutely regioselective mono-deacetylation which is difficult to achieve by methods known from literature (e.g. by treatment with aqueous alkaline bases), however.
One embodiment of the present invention is a process for converting trimethylhydroquinone diacetate (TMHQ-DA) into trimethylhydroquinone-1-monoacetate (TMHQ-1-MA) by contacting TMHQ-DA with a lipase to effect an enzymatic monosaponification of the TMHQ-DA.
Another embodiment of the present invention is a process of making (all-rac)-xcex1-tocopherol acetate having the steps of reacting trimethylhydroquinone diacetate (TMHQ-DA) with a lipase to form trimethylhydroquinone-1-monoacetate (TMHQ-1-MA), followed by reacting the TMHQ-1-MA with isophytol or an equivalent thereof to form (all-rac)-xcex1-tocopherol acetate.
A further embodiment of the present invention is a method of making (all-rac)-xcex1-tocopherol having the steps of reacting trimethylhydroquinone diacetate (TMHQ-DA) with a lipase to form trimethylhydroquinone-1-monoacetate (TMHQ-1-MA), reacting the TMHQ-1-MA with isophytol or an equivalent thereof to form (all-rac)-xcex1-tocopherol acetate, and deacetylating the (all-rac)-xcex1-tocopherol acetate to form (all-rac)-xcex1-tocopherol.
It has now been found that TMHQ-DA can be absolutely regioselectively converted into TMHQ-1-MA by subjecting the TMHQ-DA to an enzymatic monosaponification by means of a lipase.
One embodiment of the present invention is a process for converting trimethylhydroquinone diacetate (TMHQ-DA) into trimethylhydroquinone-1-mono-acetate (TMHO-1-MA) by contacting TMHQ-DA with a lipase to effect an enzymatic monosaponification of the TMHQ-DA.
In a preferred embodiment of the present invention the lipase is immobilised on a solid carrier material. Said carrier material can be a hydrophobic carrier, e.g. a polypropylene carrier such as ACCURELO(copyright) MP1001, (Membrana GmbH, Obernburg, Germany). A carrier of a different nature, namely the alkaline catalyst carrier CELITE(copyright) (chemical composition: 87% SiO2, 0.9% CaO, 6.1% Al2O3, 1.6% Fe2O3, 1.6% Na2O+K2O; pH (10% suspension, 25xc2x0 C.) =8.5) which is often used for the immobilization of enzymes, did not give a satisfying performance of the immobilized enzyme, however.
Lipases which are suitable for the purposes of the present invention include those belonging to enzyme class EC 3.1.1.3.
Among the various lipases which are available on the market the following, in particular, have proved to be particularly efficient for the purposes of the present invention: Thermomyces lanuginosus lipase (TLL); Mucor mihei lipase (MML); Alcaligenes spec. lipase (ASL); Candida rugosa lipase (CRL); Candida antartica (fraction B) lipase (CAL(B)); and Pseudomonas spec. lipase (PSL), e.g. Pseudomonas fluorescens lipase (PFL). Preferred lipases are PSL, PFL and TLL; with TLL being particularly preferred.
The enzymatic monosaponification of the invention is conveniently carried out in a hydrophobic solvent, e.g. in 1-methyl-2-pyrrolidone or, particularly, in an ether solvent such as tert.-butyl methyl ether, butyl ether, methyl 2-methyl-2-butyl ether or the like, or mixtures thereof, with tert.-butyl methyl ether being particularly preferred.
Conveniently from about 0.01 to about 99.5 vol %, preferably about 0.03 to about 20 vol %, more preferably about 0.09 to about 5 vol % of water or buffer, such as phosphate buffer, may be added to the ether solvent. Ethanol may be present in a concentration of up to 1%.
Tetrahedron 56 (2000) 317-321 describes, inter alia, the selective monosaponification of 2-methyl-1,4-diacetoxynaphthalene into the corresponding 1-acetoxy-4-hydroxy compound by means of the free enzyme PSL in tert.-butyl methyl ether in the presence of water. When repeating this experiment over a time up to 185 hours it was found, however, that inconsistent results were obtained. Furthermore, when treating TMHQ-DA with the free enzyme PSL under the same reaction conditions over a time of up to about 300 hours, the initial reaction rate was only about one third. As against that the monosaponification of TMHQ-DA by means of immobilized PSL over a time of  less than 100 hours resulted in an almost quantitative conversion, and similar results were obtained with immobilized PFL and immobilized TLL.
The reaction rate of the monosaponification of the present invention normally increases with increased reaction temperatures. The maximum temperature is, of course, limited by the boiling point of the solvent (55xc2x0 C. in the case of tert.-butyl methyl ether) but still higher temperatures can be achieved when performing the enzymatic monosaponification under pressure. With respect to some of the lipases, particularly TLL, the temperature may be raised up to about 60 to about 80xc2x0 C.
The enzymatic monosaponification of the invention is thus conveniently carried out in a temperature range of from about 4 to about 80xc2x0 C., preferably in the range of from about 20 to about 75xc2x0 C.
The ratio of enzyme, both free and immobilized, to the substrate (TMHQ-DA) can vary in a rather broad range, conveniently of from about 0.001 g/g to about 10 g/g, preferably from about 0.01 to about 0.2 g/g.
The ratio of the substrate (TMHQ-DA) to the solvent can likewise vary in a rather broad range, conveniently of from about 0.001 g/g to about 100 g/g, preferably from about 0.01 g/g to about 0.8 g/g.
When the enzyme is immobilized on an appropriate carrier the monosaponification of the invention may be performed continuously, e.g. in a fixed-bed reactor or a continuous stirred tank reactor, instead of batch-wise.
As mentioned earlier, the TMHQ-1-MA obtained by the enzymatic monosaponification of the invention can be converted into (all-rac)-xcex1-tocopheryl acetate, e.g. by reaction with isophytol. If (all-rac)-xcex1-tocopherol should be present in the crude product, such (all-rac)-xcex1-tocopherol can, if desired, be converted into its acetate by acetylation, e.g. by means of acetic anhydride. Another embodiment of the present invention is a method of making (all-rac)-xcex1-tocopherol acetate by converting trimethylhydroquinone diacetate into trimethylhydroquinone-1-monoacetate by means of a lipase, and condensing the trimethylhydroquinone-1-monoacetate with isophytol or an equivalent.
Another embodiment of the present invention is a method of making (all-rac)-xcex1-tocopherol acetate having the steps of reacting trimethylhydroquinone diacetate (TMHQ-DA) with a lipase to form trimethylhydroquinone-1-monoacetate (TMHQ-1-MA), followed by reacting the TMHQ-1-MA with isophytol or an equivalent thereof to form (all-rac)-xcex1-tocopherol acetate.
A further embodiment of the present invention is a method of making (all-rac)-xcex1-tocopherol having the steps of reacting trimethylhydroquinone diacetate (TMHQ-DA) with a lipase to form trimethylhydroquinone-1-monoacetate (TMHQ-1-MA), reacting the TMHQ-1-MA with isophytol or an equivalent thereof to form (all-rac)-xcex1-tocopherol acetate, and deacetylating the (all-rac)-xcex1-tocopherol acetate to form (all-rac)-xcex1-tocopherol.
The following examples are provided to further illustrate the process of the present invention. These examples are illustrative only and are not intended to limit the scope of the invention in any way.